Fuel cells were invented in 1839 by Sir William Grove.
A fuel cell is an electrochemical device which directly combines a fuel and an oxidant such as hydrogen and oxygen to produce electricity and water. It has an anode and a cathode spanned by an electrolyte. Hydrogen is oxidized to hydrated protons on the anode with an accompanying release of electrons. At the anode, oxygen reacts with protons to form water, consuming electrons in the process. Electrons flow from the anode to the cathode through an external load, and the circuit is completed by ionic current transport through the electrolyte.
Fuel cells do not pollute the environment. They operate quietly, and they have a potential efficiency of ca. 80 percent. Virtually any natural or synthetic fuel from which hydrogen can be extracted--by steam reforming, for example--can be employed.
A variety of electrolytes have been proposed. These include: aqueous potassium hydroxide, concentrated phosphoric acid, fused alkali carbonates, and stabilized zirconium oxide. Molten carbonate fuel cell (MCFC) power plants are of particular interest. A MCFC power plant, for example, offers cost savings and increased efficiency in converting natural gas to electrical energy in comparison to other available techniques for accomplishing this goal such as using this abundantly available gas to fuel a gas turbine engine (potential conversion efficiency of 30%). Because of cost, performance, and endurance considerations, the basic components of a MCFC fuel cell must be: easily manufactured by simple scalable techniques, stable in the fuel cell, and able to meet threshold performance levels.
Of particular importance in these respects is the "electrolyte structure" of a MCFC. This consists of: (1) a porous matrix formed by packing submicron inert particles such as LiAlO.sub.2, and (2) a carbonate phase which is retained within the pores by capillary action.
Until the early 1980's, hot pressing was used to make the electrolyte structures for molten carbonate fuel cells. The electrolyte powder was prepared by repeated blending and firing of submicron size LiAlO.sub.2 and Li.sub.2 CO.sub.3 -K.sub.2 CO.sub.3 mixtures evenly distributed in a steel die having the appropriate cavity size and geometry and then hot pressed at temperatures of 400.degree. to 500.degree. C. and pressures of 2000 to 5000 psi. The resulting product, called a hot-pressed tile, was directly used in fuel cells. The hot pressing process involves lengthy preparation of the needed electrolyte powder and a closely-controlled hot pressing technique. It is not practical for preparing large-scale multi-cell stackable components.
Another technique for making the electrolyte structures of MCFC's that has heretofore been proposed is tape casting. This approach, as described in U.S. Pat. No. 4,411,968, employs a polyvinyl butyral binder which leaves carbon residues when the cast structure is thermally treated to develop a porous, inert, matrix for the electrolyte structure. The presence of carbon is undesirable because it may inhibit wetting by the molten electrolyte. That may result in ineffective wetting of the lithium aluminate matrix by the electrolyte.
In yet another process for fabricating fuel cell electrolyte matrices, discrete and uniformly sized submicron matrix particles are coated with 10 volume percent carbonate. The resulting powder is milled, mixed with a plastic binder, and rolled into a thin sheet. Mixing and rolling are carried out hot--at 150.degree. and 135.degree. C., respectively. The resulting matrix has a 45% to 55% porosity and a mean pore size of 0.3 to 0.6 microns.
The roll-milling action leaves large interagglomerate pores in the matrix, and these have to be plugged--e.g., by using a blend of large and small particles. Also, the roll-milling process uses a polybutylene binder which melts at 125.degree. C. and has a viscosity of 25,000 to 35,000 cp at 135.degree. C. Paraffin wax is added to this binder as a lubricant. With a commercial LiAlO.sub.2 powder added, the mixture has the viscosity of a thick glue at 150.degree. C. This hampers efficient inter-particulate mixing, which is critical as the success of the matrix produced by the hot-roll milling technique is dependent on the homogeneity and uniformity of the powder/binder mixture.